Disposable absorbent articles have found widespread use and acceptance. They are frequently constructed using absorbent structures containing comminuted wood pulp and in many cases superabsorbent polymer to provide the necessary absorbent function. In many applications, such as disposable sanitary napkins, diapers, and adult incontinent devices, this absorbent structure constitutes a core which is contained in a generally non-absorbent envelope consisting of some type of liquid permeable coverstock on one side, and an impervious film on the other side. In other applications, such as some type of absorbent food pads, the covering on both sides of the core is liquid permeable, such as using perforated films. In other applications such as mops and wipers, the envelope, if present in the design, functions to modify the surface for functions such as scrubbing or strength. Alternatively, these features and functions are performed by the core itself.
Absorbent members can be made concurrently with the rest of the article or can be pre-made as a roll good and supplied to the converting machine as a raw material. In this case, airlaid composite materials comprising cellulose fibers are frequently used to make absorbent members. In many applications, there is a need for the absorbent member to exhibit wet integrity, so that it does not break apart when the normal mechanical stresses of use are applied to the article after wetting. In these end uses, the airlaid composite material then needs to exhibit wet strength.
There are several functional aspects to wet strength. In most cases, the absorbent core is in the form of a flat sheet, and there is a need for wet tensile strength, in which the sheet resists tearing when tensile forces are applied in-plane to the wetted core. In another important aspect of wet strength, the absorbent core is laminated to envelope members that are non-absorbent and have good wet strength. In order for these envelope materials to support the wet core, the surface of the core, which is attached to the envelope, must remain integral to the remainder of the core through its thickness as in-plane shearing forces are applied, preventing separations from occurring within the core. In other aspects of this type of construction, the core is laminated to envelope members on opposite faces of the core, which then have forces applied that make these envelope members move relative to one another, again applying in-plane shearing forces to the core that they both are joined to. Wet shear strength in the absorbent member is also important in wipes and mop applications, in which scrubbing forces are applied to the working face of the absorbent and can cause internal shearing failure of the absorbent member if the wet shear strength is not sufficient. Additionally, wet strength can help these welted working faces of the absorbent avoid being scuffed and tom under friction.
In the case of airlaid core materials, which are typically introduced as roll goods or otherwise introduced from a package to the converting process for the absorbent article in question, there are several widely practiced methods for producing materials with substantial wet tensile and shear strength.
The airlaid process, widely practiced commercially, consists of two principal steps. The first step involves depositing the various fibers and other materials into a uniform continuous web. The second step is the bonding process, in which this web is bonded and given the mechanical properties of the final airlaid nonwoven material.
U.S. Pat. No. 4,600,462 teaches a process in which an unbonded web comprising cellulose fibers is sprayed with a latex binder, such as a latex of EVA (ethylene vinyl acetate) or other binding agent and water. The water distributes the binder to the fiber surfaces through the thickness of the web and then heat is applied to remove the moisture and set the binder. Various forms of latex bonded airlaid (termed “LBAL” in the industry) using similar processes are widely produced commercially and exhibit good mechanical properties including wet strength. While the complexities of mixing and introducing the latex, and of handling a fragile unbonded web that is wetted with latex, have largely been dealt with in a satisfactory manner in commercial production, the energy consumption required to dry the moisture and set the binder is very high. This not only incurs a costly energy input, but also requires a very capital-intensive through-air oven in order to provide the required energy input to the moving web at rates fast enough to support good production rates. There is a need for a process that provides good wet strength while avoiding the energy and capital costs and complexity of LBAL.
U.S. Pat. No. 5,231,122 teaches an airlaid composite comprising cellulosic fibers and two thermoplastic materials, each having different melting temperatures, at least one of which is a fiber. In one embodiment, the low-melting temperature thermoplastic is in the form of a sheath around fibers comprising the high-melting temperature thermoplastic. An unbonded airlaid web comprising these materials is then heated and brought to a temperature that melts the sheath material only, which forms bonds between the fibers in the airlaid material. In commercial practice of a process similar to this, it has been found that this material is dusty, and small amounts of latex binder are frequently added to the surfaces prior to bonding the web. This process, called Multibonded Airlaid (“MBAL” in the industry) is widely practiced commercially, producing airlaid webs that have good wet shear and wet tensile strength. As with the previous example, the energy input required to activate the bonding is still very high, incurring significant costs and complexity in terms of energy usage and capital involved with the large ovens necessary to impart this energy to the web at high production speeds.
In a variant of this process, U.S. Pat. Nos. 4,425,126 and 4,129,132, to Butterworth, et al., describe a fibrous material formed by combining thermoplastic fibers and wood pulp, heat fusing the combined fibers, and thereafter depositing a binder on the heat-fused web. Because the fibers are heat-fused prior to adding the binder, individual binder coated fibers for mixing with other fibers are not produced by this process. The same issues of energy consumption and complexity exist as with the previous examples.
There is a need for a bonding process for airlaid webs comprising cellulosic fibers that has a smaller energy input requirement and avoids the need for the large ovens required for LBAL and MBAL processes.
U.S. Pat. No. 5,866,242 teaches an airlaid material, sometimes referred to as a Hydrogen Bonded Airlaid, comprising cellulosic fibers, and optionally superabsorbent polymer that is bonded using heat and pressure to form hydrogen bonds. In commercial practice, this technology uses a heated calender roll to apply the pressure and heat required to form hydrogen bonds between the fibers. Compared to LBAL and MBAL, the energy input requirement to form strong bonds is significantly less using this method. This arrangement is much simpler to operate, and has significantly less energy consumption and requires much less capital expenditure than the ovens used in the LBAL and MBAL processes.
Hydrogen Bonded Airlaid, however, has relatively low wet tensile or shear strength, particularly if there is SAP present, which acts to debond the web as it hydrates and swells upon welting.
There is therefore a need for a method of producing a material with improved wet strength requiring a lower energy input, thus desirably allowing the use of the heated calender means used in a Hydrogen Bonded Airlaid process, thus avoiding the complexity, capital cost and energy use of hot air bonding ovens. It is additionally desirable to achieve this as an add-on to a hydrogen bonded airlaid operation with a minimum of additional capital.
In an attempt by the applicants to accomplish this, bi-component fibers, comprising two thermoplastic materials with differing melting points, were blended with cellulosic fibers in a hydrogen bonded airlaid composite made on a commercial production machine using a heated calender bonding station. It was found that the calender station, operating at 170° C. that provided high production rates for the hydrogen bonded airlaid process, did not provide enough heat energy input to effectively activate the bi-component fiber unless the line was slowed to an undesirably low speed.
In a separate similar attempt by the applicants, fusible binders were dispersed in an airlaid hydrogen bonded composite in the form of finely divided powders, and bonded using a heated calender bonding station. Both polyethylene and EVA binders were tried. As in the previous trial, the calender on the production machine did not provide enough heat to effectively fuse the powders providing improved wet strength unless the line was slowed to an undesirably slow speed. The applicants believe that the heat of vaporization of moisture present in the web is causing the web to resist exceeding 100° C. until most of the moisture is driven off. These fusible binders and bi-component fibers activate at temperatures above 100° C.
There is therefore a need to introduce a wet strength agent to the hydrogen bonded airlaid that activates and forms wet-strength bonds at temperatures well below 100° C.
Notably, wax is a material that has a melting temperature in a range very favorable for processing using a heated calender. The use of wax in paper products is widely practiced. Wax is frequently used as a wet strength agent, and as a barrier to water penetration in structural materials, such as corrugated boxes. Alternatively, wax is frequently used as a barrier to liquid absorption, such as is taught in U.S. Pat. Nos. 3,654,064, 5,399,366, or 4,601,938. In some applications, wax is intended to be distributed through a paper product at very low add-on levels, which is accomplished by applying water emulsions of wax, and then later removing the water. This is taught by U.S. Pat. Nos. 7,300,547 and 4,987,632, both hereby incorporated by reference, as well as others, but in each case, the intended function of the wax is to inhibit or block water absorption. Similar to an LBAL process, it is presumed using this method in an airlaid material would require sufficient energy input and capital in the form of a large oven in order to evaporate the moisture from the emulsion and produce such a material at high line speeds. In U.S. Pat. No. 7,228,586, a wax coated paper is specified as an absorbent member in a multilayer scrubbing pad, however, there is no indication of high absorbency, particularly given the absence of SAP.
Thus, a need exists for a method of wax application for a hydrogen bonded airlaid material that confers improved wet strength while maintaining most of the absorbent capacity of the material.